For a number of years, specialized mechanical devices called "bolts" have been used to support or reinforce rock structures, primarily roof structures in underground mines. They are believed to work in two ways. First, the bolt can be anchored in strong rock which has the ability to support the load of weaker rock below it. Here, the weak rock in the immediate ceiling is essentially suspended by the bolt from the stronger rock above it. Second, and perhaps the more prominent effect, the rock between the two ends of the bolt is compressed, exerting a horizontal force which, essentially, builds a beam of rock which forms the roof of the mine. Theoretically, the more tension on the bolt, the stronger the beam, and less chance of roof fall.
Although the mechanism of rock bolting is not yet fully understood, the method of use is relatively straight forward. In its simplest form, rock bolting involves drilling or otherwise forming one or more holes (often they are generally vertical) in an overhead rock formation. The diameter of each hole is generally small, typically on the order of 2-6 centimeters. In the United States, hole diameters are typically 13/8 inch to 13/4 inch. A specially designed rock bolt is then inserted into the hole. A typical rock bolt involves a partially threaded shaft, the inner (or buried) end of which is engaged (e.g. threadably engaged) with some form of operable anchoring device (e.g. an expandable anchor having several moving parts) which can be manipulated to anchor the rock bolt in the hole. Rock bolts are typically 100-300 centimeters in length, although much longer bolts (e.g. 1500 cm) are used in some civil engineering projects. After the rock bolt is pushed into its hole, the bolt rod is rotated about its longitudinal axis or otherwise manipulated so that the anchoring device is expanded or forced outwardly against the inside wall of the hole. The outer (or exposed) end of the rock bolt is firmly fastened over the end of the hole by some type of collar or bearing plate or similar device which is held in place by means of an integral head or a threaded nut on the outer end of the shaft of the rock bolt. As a consequence, the bolt is firmly anchored in the hole by the wedging action that has occurred at the upper end of the rock bolt and the bolt itself is placed under considerable tension as the nut or head on the lower end is tightened. By careful placement and proper tightening of a series of rock bolts, it is possible to support or reinforce a variety of rock structures. If desired, the rock bolt can be protected against chemical attack and also more securely fastened into the rock formation by pumping or otherwise introducing a hardenable grout into the hole, usually along the entire length of the bolt after the bolt has been tensioned.
The concept of rock bolting can be better understood by reference to several of the drawings (in which the straight diagonal lines are used to indicate rock formations and not fracture lines).